EVA blend compositions suitable for RF welding applications

ABSTRACT

Disclosed are films obtained from blends of ethylene copolymers with organic acid salt-modified potassium ionomeric copolymers. The films of the present invention can be welded using RF energy.

This is a division of application Ser. No. 12/077,585, filed Mar. 20,2008, now U.S. Pat. No. 7,875,680 which is a continuation of priorapplication Ser. No. 11/127,563, filed May 12, 2005 now abandoned whichclaims the benefit of U.S. Provisional Application No. 60/570,550, filedMay 12, 2004, now expired, the entire contents being incorporated hereinby reference.

FIELD OF THE INVENTION

This invention relates to polymer compositions that absorb radiofrequency energy. More particularly, this invention relates to filmsobtained from polymer compositions that can be welded using radiofrequency energy.

BACKGROUND DISCUSSION AND RELATED ART

In packaging applications, it can sometimes be desirable to seal a filmpackage without the use of a separate adhesive film layer on thepackage. Such a sealing process can be obtained by using materials thatcan be self-adhered, or materials that can be made to adhere to or toadhere to another material. One such process is a process by which radiofrequency (RF) energy is applied to a suitable material, and afterabsorption of the RF energy the material can subsequently beself-adhered, or adhered to different substrates to form a seal. Thisprocess is known as RF welding, and suitable materials for such aprocess are known in commercial practice. Polyvinyl chloride (PVC), forexample, is a material that can be susceptible to RF energy and be verysuitable for RF welding. PVC, however, is not an acceptable material forwidespread use in some areas of the world and alternate materials areneeded to perform the function of PVC in those regions where PVC is notdesirable.

Selecting suitable alternate materials for use in packaging applicationsthat use the RF welding technique is not a trivial exercise. Inselecting or developing suitable alternative materials, at least three(3) criteria need to be met. First, the material must provide goodphysical and/or mechanical characteristics that are satisfactory for apackage or a component thereof. Second, a suitable polymer film shouldbe susceptible, that is should absorb, RF energy. Third, the polymerfilm should be capable of forming a bond, weld, or seal, when subjectedto RF energy under the proper conditions of pressure and time.

Various polymers suitable for use in packaging applications are wellknown. For example, polyolefins such as polyethylene and polypropyleneare useful in packaging. Some polymeric materials are known to besusceptible to RF energy. However, other materials may not be suitablefor use in the applications described herein due to prohibitive cost orother problems. For example, some materials can be susceptible to RFenergy but are not capable of forming a seal, or weld. Some materialscan require more stringent process conditions, or require that acomponent be increased to the detriment of other physical/mechanicalproperties. The susceptibility of EVA, for example, is highly dependenton the vinyl acetate content. Typically ethylene (meth)acrylic acidcopolymers and their ionomers are not RF susceptible.

Mixtures of ionomers and polar solvents such as glycerol can producecompositions that absorb RF energy. However, use of solvents havingconsiderable vapor pressure at extrusion temperatures can lead to otherproblems, such as fogging during processing. Also, use of polar solventscan be undesirable for environmental reasons.

It can be desirable to have a composition that absorbs RF energy, andcan be sealed using RF welding, while yet having physical and mechanicalproperties suitable for use in packaging applications.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a film that absorbs radiofrequency (RF) energy wherein the film comprises:

(a) a blend comprising (i) at least one E/X/Y copolymer where E isethylene, X is a C₃ to C₈ α,β ethylenically unsaturated carboxylic acid,and Y is a softening comonomer selected from alkyl acrylate and alkylmethacrylate wherein the alkyl groups have from one to eight carbonatoms, wherein X is about 2-30 weight % of the E/X/Y copolymer and Y isabout 0-40 weight % of the E/X/Y copolymer, wherein the acid component(X) can be present in either the fully neutralized carboxylate salt formor the partially neutralized carboxylate salt form, and

(ii) one or more organic acids or salts thereof; wherein the combinedcarboxylate salt functionalities of the blend are at least partiallyneutralized by potassium; and

(iii) one or more polar compounds selected from polyols and polyesters;and,

(b) optionally at least one other polymeric component, wherein the filmabsorbs radio frequency (RF) energy.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention is a polymeric film obtainedfrom a blend comprising an acid copolymer ionomer and a fatty acid salt.A film of the present invention is at least about 0.001 mil thick.Preferably the film is from at least about 1 mil thick to about 25 milthick.

Films of the present invention comprise acid copolymer ionomers(“ionomers”), which are copolymers well known in the polymer art.Ionomers are ionic copolymers formed by copolymerization of anα-olefin—such as ethylene, propylene, 1-butene, and other 1-unsaturatedhydrocarbons—with an unsaturated carboxylic acid, such as for exampleacrylic acid, methacrylic acid, or maleic acid, and optionally softeningcomonomers, in which some portion of the acidic groups in the copolymeris neutralized with metal ions such as sodium or zinc. As used herein,the term “copolymers” can describe polymers obtained by copolymerizationof at least two different monomer species. The term “terpolymer” can beused to specifically describe a copolymer that is obtained fromcopolymerization of three different monomer species. At least one alkalimetal, transition metal, or alkaline earth metal cation, such aslithium, sodium, potassium, magnesium, calcium, or zinc, or acombination of such cations, can be the counterion that neutralizes thecarboxylate anion of the salt. Particularly preferred are ionomers thatare at least partially neutralized with potassium cations. Terpolymerscan also be made from an olefin such as ethylene, an unsaturatedcarboxylic acid and a softening comonomer such as an alkyl(meth)acrylate providing “softer” resins which can be neutralized toform softer ionomers.

A blend useful in the practice of the present invention comprises atleast one E/X/Y acid copolymer composition where E is ethylene, X is aC₃ to C₈ α,β ethylenically unsaturated carboxylic acid, and Y is asoftening comonomer selected from alkyl acrylate and alkyl methacrylatewherein the alkyl groups have from one to eight carbon atoms, wherein Xis about 2-30 weight % of the E/X/Y copolymer and Y can be present in anamount of from about 0 to about 40 weight % of the E/X/Y copolymer. Theacid copolymers can be fully or partially neutralized by known andconventional means to provide acid copolymer ionomers (ionomers).Ionomers suitable for use in the practice of the present invention areknown and commercially available from E. I. du Pont de Nemours andCompany under the tradename of Surlyn®.

Additionally, films of the present invention comprise an organic acid ora salt thereof (organic salt). In the practice of the present invention,reference to an organic acid can also refer to the salt of the acid, ora mixture of the acid and the salt. Blends useful in the practice of thepresent invention include at least about 5 wt % of the organic acid.Preferably the blends include from about 5 wt % up to about 50 wt % ofan organic acid. More preferably the blends include at least about 10 wt% of the organic acid, even more preferably at least about 20 wt %, andmost preferably at least about 30 wt % of the organic acid and/or saltthereof. The counter ions of the organic salts are preferably at leastin part potassium ion. Preferably, greater than 70% of all the acidcomponents in the blend are neutralized, more preferably greater than90% are neutralized. Most preferably, 100% of all the acid components inthe blend are neutralized.

The organic acids of the present invention are particularly those thatare non-volatile and non-migratory, for example acids comprising fromabout 6 to about 38 carbon atoms. Non-limiting, illustrative examples oforganic acids are stearic acid and isomers thereof, oleic acid andisomers thereof, erucic acid and isomers thereof and behenic acid andisomers thereof. Stearic and oleic acids and their isomers arepreferred. Even more preferred are branched isomers of suitable organicacids such as branched isomers of stearic and oleic acids, such asisostearic acid and isooleic acid, for example. A more completedescription of suitable acids and blends for use in the presentinvention can be found in U.S. Published Patent Application 2005/0037216A1, published Feb. 17, 2005, fully incorporated herein by referencethereto. Preferably, the organic acids are neutralized and have acounterion selected from the group consisting of potassium (K), sodium(Na), lithium (Li), rubidium (Rb), and magnesium (Mg) ions. Morepreferably the counter ion is K.

In a preferred embodiment, the present invention includes as the organicacid an isomer of a C₆ to C₃₈ organic acid. Particularly preferred areisomers of oleic acid such as 2-methyl oleic acid (isooleic acid), andisomers of stearic acid such as 2-methyl stearic acid (isostearic acid).Also preferred for use in the practice of the present invention arefunctionalized fatty acids such as, for example, amino- andhydroxyl-functionalized fatty acids. Hydroxystearic acids such as12-hydroxy stearic acid, for example, can be a preferred functionalizedfatty acid.

A suitable blend for use in the practice of the present invention cancomprise optional polymeric components. Such optional components caninclude a second ionomeric copolymer, including dipolymer ionomers andterpolymer ionomers; and/or thermoplastic resins. The thermoplasticresins would, by way of non-limiting illustrative examples, includethermoplastic elastomers, such as polyurethanes, poly-ether-esters,poly-amide-ethers, polyether-ureas, PEBAX (a family of block copolymersbased on polyether-block-amide, commercially supplied by Atochem);styrene-butadiene-styrene (SBS) block copolymers; styrene(ethylene-butylene)-styrene block copolymers; polyamides (oligomeric andpolymeric); polyesters; polyvinyl alcohol; polyolefins includingpolyethylene, polypropylene, ethylene/propylene copolymers; ethylenecopolymers with various comonomers, such as ethylene/vinyl acetate,ethylene/(meth)acrylates, ethylene/(meth)acrylic acid,ethylene/epoxy-functionalized monomer, ethylene/CO, ethylene/vinylalcohol (polyols); functionalized polymers with grafted maleic anhydridefunctionality and epoxidized polymers; elastomers, such as EPDM,metallocene catalyzed PE and copolymer, ground up powders of thethermoset elastomers. The optional second polymer component can be ablend comprising at least one any of these materials, including thosematerials that are not susceptible to RF radiation.

Preferably the second component is a copolymer of ethylene such as forexample, ethylene copolymers with various comonomers, such asethylene/vinyl acetate, ethylene/(meth)acrylates, ethylene/(meth)acrylicacid and ionomers thereof, ethylene/epoxy-functionalized monomer,ethylene/CO, ethylene/vinyl alcohol, or a blend comprising at least oneof these. More preferably, the second polymer component is a selectedfrom the group consisting of: ethylene vinyl acetate (EVA);ethylene/alkyl acrylates; ethylene/(meth)acrylic acid and ionomersthereof; or a blend comprising at least one of these.

If included, the amount of the optional polymer component is preferablyincluded in an amount of 99% by weight to about 1% by weight of thewhole potassium ionomer composition. Preferably, the optional polymercomponent is included in an amount of from about 95 wt % to about 50 wt%, more preferably in an amount of from about 93 wt % to about 70 wt %.

The films of the present invention can be used in monolayer ormultilayer structures. The films of the present invention can be usefulin such packaging applications as film wraps, containers, and lidswithout limitation thereto.

Examples of the unsaturated carboxylic acid include acrylic acid,methacrylic acid, fumaric acid, maleic anhydride, monomethyl maleate,monoethyl maleate. Particularly preferred are acrylic acid and/ormethacrylic acid. Examples of polar monomers that can serve ascopolymerization components include vinyl esters such as vinyl acetateand vinyl propionate; unsaturated carboxylic acid esters such as methylacrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutylacrylate, n-hexyl acrylate, isooctyl acrylate, methyl methacrylate,dimethyl maleate and diethyl maleate; carbon monoxide. In particular,unsaturated carboxylic acid esters are suitable copolymerizationcomponents.

EXAMPLES

Materials Used

Elvaloy AC1224 is a copolymer blend of ethylene vinyl acetate andethylene methacrylic acid copolymers, each available from E. I. du Pontde Nemours and Company (DuPont).

Glycerol monostearate (GMS) (or potentially esters of polyglycerols), orsorbitol monostearate can be useful in the practice of the presentinvention.

General Procedures

Films of blends were prepared by blending copolymers of Elvaloy® AC1224,commercially available from DuPont, with different levels of the RFenhancers, between 5% and 15%, and extruding the blends on a prismextruder to produce film between 6 and 12-mil thick, and these weretested for weldability. Two layers of the film were backed by Kapton®sheet and placed between the electrodes of an RF source, under a slightpositive pressure and exposed for generally 2 seconds.

Dielectric measurements can be used to measure RF adsorption.Alternatively a simple relative method is to measure the effect ofintroducing a material into the capacitance area of an alternatingsignal and measure the effect on the signal width or ‘Q value’(frequency divided by width). A lowering of the Q value indicatesadsorption, and since this is relative for a given circuit a percentagelowering of Q is a useful value. Welding experiments can be carried out,by subjecting a pair of film samples to an alternating field, for afixed time, under a certain pressure.

TABLE 1 % Reduction in Q PVC (average of 9 12.4 +/− 3 determinations) 9mil PVC filled, heavily plasticized 18.8 +/− 2 (average of 3) 9 milElvaloy*AC1224 19.7 +/− 3 (average of 5) 6 mil Elvax*3170, 18 mil 18.8Surlyn*8140  8 Surlyn*8150 + 20% glycerol 6 mil 60 (up to 50% humidity)95 (75% humidity) Surlyn*8150 + 20% GMS 13 (up to 50% humidity)Surlyn*8140 + 30% potassium 78 (up to 50% humidity) 12-hydroxystearate96 (75% humidity) Surlyn*8140 + 20% potassium 89 (25% humidity)iso-stearate 95 (75% humidity) A selection of the results obtained were:PVC 4 Highly plasticized, 4 filled PVC Elvaloy*AC1224 0 AC1224 + 10% 25%humidity 4 (Surlyn ® + glycerol) 75% humidity 4 AC1224 + 10% 25%humidity 1 (Surlyn ® + GMS) 75% humidity 2 AC1224 + 10%) 25% humidity 3(Surlyn ® + K hydroxy stearate) 75% humidity 3 Elvax*3170 20% humidity 1(at 3-4 kV/3 s) 3 (at 4.5 kV/3 s) Elvax*3170 + 10% 50% humidity 3 (at3.2 kV/2.5 s) (Surlyn ® + K iso-St*) Elvax*3170 + 20% 50% humidity 4 (at2.8 kV/1 s) (Surlyn ® + K iso-St) Elvax*3170 + 30% 50% humidity 4 (at 2kV/1 s) (Surlyn ® + K iso-St) % Q-value reduction SiSt¹ (25% RH) 89%(50% RH) 95% (75% RH) 96% Ex. A1: Elvax 3170/10% SiSt (25% RH) 15% (50%RH) 59% (75% RH) 43% Ex. A2: Elvax 3170/20% SiSt (25% RH) 53% (50% RH)80% (75% RH) 49% Ex. A3: Elvax 3170/30% SiSt (25% RH) 86% (50% RH) 89%(75% RH) 90% Welding test: Welding rating Ex. A1: 3 (at 3.2 Kvolt/2.5sec.) Ex. A2: 4 (at 2.8 Kvolt/1 sec.) Ex. A3: 4 (at 2.0 Kvolt/1 sec.)Elvax 3170 1-2 (at ~3.5 Kvolt/3 sec.) The weld performance was rated onan arbitrary scale. A value of 4 is standard for conventional weldablecompositions. Factors such as film thickness, time and voltage canaffect the results. 0 - Negligible adhesion between the films 1 -Noticeable adhesion 2 - Sufficient adhesion to deform the film onpeeling 3 - Tearing of the film instead of peeling along some part ofweld 4 - Tearing of the film with no peeling along entire weld*potassium isostearate ¹Surlyn ® 8140/20% K iso-St

1. A process for preparing a radio frequency welded packaging materialcomprising the steps of A. providing a first film substrate having athickness of at least about 0.0001 mil to about 25 mil that absorbsradio frequency energy consisting essentially of
 1. a first componentcomprising a mixture of a. at least one polymer selected from the groupconsisting of E/X/Y copolymers, ionomers of E/X/Y copolymers andmixtures thereof, where E is ethylene, X is a C₃ to C₈ α,β-ethylenicallyunsaturated carboxylic acid and Y is a comonomer selected from the groupconsisting of alkyl acrylates and alkyl methacrylates having alkylgroups of from one to eight carbon atoms, wherein said copolymerscomprise about 2-30 weight %, based on the weight of the E/X/Ycopolymer, of copolymerized units of X and about 0-40 weight %, based onthe weight of the E/X/Y copolymer, of copolymerized units of Y; and b. amaterial selected from the group consisting of organic carboxylic acidshaving from about 6 to about 38 carbon atoms, salts of said carboxylicacids and mixtures thereof; wherein at least a portion of the combinedcarboxylic acid groups present in said polymer and said material of saidfirst component are neutralized by potassium ions; and
 2. a secondcomponent in an amount of 50-95 wt. %, based on the total weight of saidfirst and second components, wherein said second component is differentfrom said at least one polymer of said first component and is selectedfrom the group consisting of polyethylenes, polypropylenes, ethylenepropylene copolymers, ethylene vinyl acetate copolymers, ethyleneacrylate copolymers, ethylene methacrylate copolymers, ethylenecopolymers having copolymerized epoxy-functionalized monomer units,ethylene carbon monoxide copolymers, ethylene vinyl alcohol copolymersand mixtures thereof; B. contacting said first film substrate with asecond substrate that is the same or different from said first filmsubstrate to form a multilayer structure; and C. exposing saidmultilayer structure, under pressure, to a source of radio frequencyenergy for a time sufficient to weld said first substrate to said secondsubstrate.
 2. A process of claim 1 wherein the second component of thefirst film substrate is present in an amount of from about 70 to about93 wt %, based on the total weight of the first and second components.3. A process of claim 1 wherein the second component of the first filmsubstrate is selected from the group consisting of polyethylenes,polypropylenes, and ethylene propylene copolymers and mixtures thereof.4. A process of claim 1 wherein the second component of the first filmsubstrate is selected from the group consisting of ethylene vinylacetate copolymers, ethylene copolymers having copolymerizedepoxy-functionalized monomer units, ethylene carbon monoxide copolymers,ethylene vinyl alcohol copolymers and mixtures thereof.
 5. A process ofclaim 4 wherein the second component of the first film substrate is anethylene vinyl acetate copolymer.
 6. A process of claim 1 wherein thesecond component of the first film substrate is an ethylene/alkylacrylate copolymer.
 7. A process of claim 1 wherein the first componentof the first film substrate comprises a mixture of a first ionomer of anE/X/Y copolymer and at least one additional ionomer of an E/X/Ycopolymer that is different from said first ionomer.
 8. A process ofclaim 1 wherein the second substrate is different from said first filmsubstrate.
 9. A process of claim 1 wherein the combined carboxylic acidgroups in the polymer and the material of the first component of thefirst film substrate are neutralized solely by potassium ions.
 10. Aprocess of claim 1 wherein the first film substrate has a thickness ofabout 6 mil to about 12 mil.
 11. A process of claim 1 wherein the firstfilm substrate is a multilayer film.
 12. A process of claim 1 whereinthe first film substrate is a film wrap or lid.